"It's such a peculiar animal," said Dennis Voeten, a palaeontology researcher at Palacky University in the Czech Republic.

"It combines different parts we knew from other groups into this one small animal."

In a study released by the journal Nature, Mr Voeten and co-authors named it Halszkaraptor escuilliei (HAHL-shka-rap-tor ES-key-lay-ee) or "Halszka" after the late Polish palaeontologist Halszka Osmolska.

Its mashup body let it run and hunt on the ground and fish in fresh water, said study co-author Paul Tafforeau.

He is a palaeontologist at the European Synchrotron in Grenoble, France, a powerful X-ray generator where numerous tests were made on the fossil.

'Is this a real, natural skeleton?'

Lead author Andrea Cau, a palaeontologist at the Geological Museum Capellini in Italy, said he was at first highly suspicious about the fossil's authenticity, both because of its appearance and the fact that the rock containing the skeleton had been smuggled out of Mongolia and left in a private collector's hands.

"I asked myself, 'Is this a real, natural skeleton, or an artefact, a chimera? If this is a fake, how could I demonstrate it?'" Mr Cau wrote in an email.

"Assuming it was a fake instead of starting assuming that the fossil is genuine was the most appropriate way to start the investigation of such a bizarre fossil."

So researchers used the synchrotron to create three-dimensional images of the fossil, which showed the creature was indeed a single animal and not a concoction built up from several sources.

For example, an arm hidden in the rock perfectly matched the visible left arm, and lines indicating growth matched up across the bones.